First work out the volume of your tank to gauge how much sea salt is needed for the aquarium. Now measure a little less than the calculated amount. (To work out tank volume in litres, measure the length x width x height in cm and divided it by 1000)
It is best to use reverse osmosis water at all times, as it will not have the impurities of tap water. Grab a mixing bucket and pour the R/O water in until, adding the amount of salt you have worked out for that amount of water. Stir the mixture well and then turn on the heater and air pump until the temperature, salt level and pH are correct before you use the water. If you are doing a 10% or less water change, the water can be used once the salt had dissolved, but if it is a larger change it is best to wait for 8 to 12 hours allowing for the sea salt to mix and the temperature to stabilise.
After the time has lapsed, measure the mixture with the hydrometer. One of two things may occur . . .
Either the SG reading will be too low or too high. If the level is too low, measure a little more salt to add to the mixture and leave it so the salt can dissolve and the temperature can stabilise. Test it again and if the same occurs, repeat this step until the water is ready.
If the SG reading is too high, add more R/O water and wait. Test again and repeat this step if the next SG reading is still too high.
Keep notes so you can quickly mix the next batch of salt water when needed because you will know the amounts already. It is suggested to regularly check the SG for the first few times until you get the hang of it and are sure that your mix is suitable for your tank.
Tap water can be used in place of RO water but you may find some problems such as more diatom algae in the tank.
Good luck and enjoy.